Sciencemadness Discussion Board

Reaction of copper(II) chloride with aluminium

LanthanumK - 1-6-2011 at 12:31

What is the formula for this reaction? It could be any of these.

3 CuCl2 + 2 Al --> 2 AlCl3 + 3 Cu
3 CuCl2 + 4 Al + 6 H2O --> 2 Al(OH)3 + 3 H2 + 2 AlCl3 + 3 Cu
6 CuCl2 + 3 H2O + 4 Al --> 4 AlCl3 + 3 Cu2O + 3 H2

I believe that it is the latter because hydrogen bubbles are emitted and the resulting red precipitate dissolves in ammonia to make a colorless solution which turns blue upon aerial oxidation. The solution resulting from the reaction is not turbid.

I have not seen the latter reaction mentioned anywhere on the Internet, though.

Any thoughts or reactions that I missed? I know that all of these reactions are technically incorrect because some of the chemicals are dissociated in solution.

hkparker - 1-6-2011 at 13:24

<a target="tab" href="http://woelen.homescience.net/science/chem/exps/cu+al/index.html">From woelen's awesome website:</a>
Quote:

The following reactions occur:

2Al(s) + 6H2O(aq) → 2Al(OH)3(s) + 3H2(g)

2Al(s) + 3CuCl42-(aq) → 2AlCl4–(aq) + 4Cl–(aq) + 3Cu(s)


The whole article is a must read.

[Edited on 1-6-2011 by hkparker]

IrC - 1-6-2011 at 13:33

2 Al + 3 CuCl2 --> 2 AlCl3 + 3 Cu is valid.

Cannot find a reference for the last two but:

AlCl3 + 3H2O = Al(OH)3 + 3HCl,
2Al + 6HCl = 2AlCl3 + 3H2,
2Al + 6 H2O = 2Al(OH)3 + 3 H2,
2Al + 3H2O = Al2O3 + 6H,
2AlCl3 . 6H2O = Al2O3 + 6HCl + 9H2O

So I wonder if stripping the oxide layer off the Al does not lead to some of these reactions while some of the others are going on as side reactions?


hkparker must have posted while I was typing this, more info is good. Reading Woelens page makes me wonder if some of these side reactions are not part of the answer to the questions he has on that page. Damn I love Woelen's work!


[Edited on 6-1-2011 by IrC]

LanthanumK - 1-6-2011 at 13:56

When I do the reaction with copper(II) chloride I don't get a turbid solution though. I read through the article and woelen gets a turbid solution, which shows that Al(OH)3 is made.

IrC - 1-6-2011 at 14:29

You did say hydrogen is coming up, the reaction 2Al + 3H2O = Al2O3 + 6H is very likely going on with fresh surfaces on the Al after other reactions have taken place. I think you are merely looking at a more complex series of events than you may at first think you have.

sternman318 - 1-6-2011 at 15:36

On Woelen's page:
Probably the aluminium/copper reaction is also fast, due to formation of a copper-aluminium electrolytic couple, which causes the aluminium to be oxidized quickly, while hydrogen from the water is produced at the copper side of the connected metal-pair.

What exactly is an electrolytic couple?

IrC - 1-6-2011 at 23:21

Two metals in contact, Al at bottom and Cu near top of the galvanic series. Like using two dis-similar metals in an electrolyte to create a battery, the Al-Cu contact sets up a voltage potential where in this case the Cu causes the Al to be eaten up faster. I am positive this is what Woelen was saying, i.e. the Al is more negative in terms of electrochemical potential. The potential difference attracts positive ions to one side and negative to the other. IIRC in effect you can say oxidation at one end and reduction at the other. Say Al is one in the series and Cu is 10. All lower metals will be accelerated in their corrosion by all metals higher up in the series. I may be explaining this poorly but read the PDF I am including, it may aid your understanding. No idea why the file does not include Mg in the chart, it would help people to understand why a Mg sacrificial anode is used in swamp coolers to prevent the cooler tank from rusting out. An unfortunate omission by the author considering the swamp cooler uses this idea more than any other device common in the desert southwest.

http://en.wikipedia.org/wiki/Galvanic_anode


Attachment: Preventing Electrolysis.pdf (11kB)
This file has been downloaded 623 times

Edit to add: since Al is 1 in the PDF think of Mg as zero, or most negative of all metals on the chart (where 0 to 12 merely indicates position in terms of relative electro-negativity, 0 being most negative and 12 most positive).


[Edited on 6-2-2011 by IrC]

Mr. Wizard - 2-6-2011 at 15:21

Quote: Originally posted by sternman318  
On Woelen's page:
Probably the aluminium/copper reaction is also fast, due to formation of a copper-aluminium electrolytic couple, which causes the aluminium to be oxidized quickly, while hydrogen from the water is produced at the copper side of the connected metal-pair.

What exactly is an electrolytic couple?


Think of it as a short circuited battery. The active material gets used up more quickly, as the two metals (usually) are in contact, allowing current to flow.